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Full text of "Handbook Of Chemical Engineering - I"

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406                               CHEMICAL ENGINEERING
of the general method was, however, well recognized and almost as soon as Steffens perfected his process of using lime, it sprang into nearly universal favor.
In carrying out the Steffens process, the diluted beet molasses is added with thorough mixing to a thick cream of freshly slaked lime (calcium hydroxide). Two important reactions take place. The sucrose forms the insoluble calcium trisacchar-ate, and the excess lime destroys any reducing sugars which may be present, and which, if separated with the sucrose adversely affect crystallization. The lime-sucrose precipitate is filter pressed and washed free of soluble salts, mixed to a stiff cream with water and the sugar set free by passing carbon dioxide into the mixture forming calcium carbonate. The sugar solution is filtered off, the calcium carbonate washed free from sugar and reburned to furnish lime and carbon dioxide for further use. The sugar solution is evaporated and crystallized along with fresh beet juice. The washings are used for mixing the lime-sucrose cream preparatory to carbonation. There .are two objections or rather difficulties with the Steffens process. The first is that it is difficult to maintain the exact conditions under which all the sucrose will be precipitated, so that there is apt to be some loss of sugar in the washings of the lime-sucrose precipitate. The second difficulty, already mentioned in connection with the osmose process, is that raffinose is equally as precipitable as sucrose and is therefore carried back into process with the sucrose. The result is that every so often the ramnose content of the molasses reaches a limiting maximum and a batch must be thrown away with the unavoidable loss of the sucrose contained.
There are plants abroad using both the strontium and the barium processes, but so far as known only one in this part of the world. This is a Canadian plant using barium carbonate as raw material. Because of the difficulty of driving off the C02 it is necessary to heat the barium carbonate in an electric furnace. The barium oxide converts readily to barium hydroxide on treatment with water, and is used as a strong solution. The addition of the beet molasses to this barium-hydroxide solution instantly produces a thick precipitate of barium trisaccharate, and by working at the correct temperature the formation of the raffinose salt is prevented. The precipitation of the sucrose is readily made complete and a recovery of 95 to 97 per cent of the sucrose in the molasses is claimed. The decomposition of the barium-sucrose precipitate is so complete that not a trace of barium is normally found in the sucrose solution. A breakage of filter cloth may sometimes allow a little barium carbonate to escape, but this is prevented from getting into process by a system of settling tanks. In spite of the high cost of the conversion of the barium carbonate into barium hydroxide, the high percentage of recovery makes the process financially profitable even when compared with the Steffens process. An interesting side light is the fact that the filtrate from the barium-sucrose precipitate is concentrated and sold for the manufacture of potassium salts for fertilizer for growing new crops of beets.
Crystallization and Purification of Ferrocyanides.—Closely allied to the above described processes for sugar is the purification and crystallization process for ferrocyanides. After leaching out the soluble ammonium salts present the insoluble iron-cyanogen compounds in the spent oxide from purifying coal gas are converted into the soluble lime salt, calcium ferrocyanide, by treatment with slaked lime and leaching. The resultant solution contains a variety of undesirable compounds, chief of which are soluble sulphates and sulphocyanides. Calcium ferrocyanide has the property of forming a very slightly soluble salt with ammonia in the presence of hydrochloric acid. In practice, in the manufacture of sodium ferrocyanide, to the calcium-ferrocyanide solution is added the crude ammonium-sulphate solution previously leached out, hydrochloric acid is added to acidity and enough ferrous sulphate to complete the precipitation.